1. Field of the Invention
The present invention relates to a printing apparatus and a data processing method.
2. Description of the Related Art
As a printing apparatus, there is an inkjet printing apparatus which performs printing by ejecting inks from an inkjet printing head on a printing medium so as to form dots. The inkjet printing apparatus has various advantages such as easiness to achieve higher-precision printing, excellence in speed and quietness, and low cost.
Some of such inkjet printing apparatuses are configured to form an image on a printing medium by using a printing head in which nozzles as printing elements for ejecting inks are arranged in a predetermined direction at a predetermined pitch, while alternately repeating a printing movement and a conveying operation. In the printing movement, printing is performed while moving the printing head in a first direction which is different from the predetermined direction. In the conveying operation, the printing medium is conveyed in a second direction intersecting the first direction. Hereinafter, for convenience, movement of a printing head is referred to as the main scanning or the print scanning, and the moving direction thereof (first direction) is referred to as the main scanning direction. On the other hand, conveyance of a printing medium is referred to as the sub-scanning, and the conveying direction thereof (second direction) is referred to as the sub-scanning direction. In such an inkjet printing apparatus, a printing method called “multipass” is applied in many cases when high quality printing is performed.
FIG. 17 is an explanatory view of the multipass printing method. In the multipass printing method, multiple times of main scanning of the printing head are performed for a unit region on the printing medium, thereby performing printing based on the printed data during the multiple times of the main scanning. In the multipass printing, a printing operation is commonly named from the number of times of the main scanning (number of passes) for the unit region. As shown in FIG. 17, a printing in which the main scanning is performed three times for the unit region (in this example, a single print region) is referred to as the 3-pass printing. Note that, the definition of the “unit region” is not limited to this. For example, a region corresponding to “a” times of a width of a nozzle pitch (where “a” is a natural number) may be defined as the “unit region”.
Note that, FIG. 17 shows that the positions of the printing head are shifted by a predetermined amount for each main scanning (from the first print scanning to the fourth print scanning), while the printing medium is fixed. However, these are illustrated for the purpose of convenience. Actually, the position of the printing head is fixed in the sub-scanning direction, and the printing medium is conveyed, between two continuous main scannings, toward the top of the drawing by a predetermined amount which is less than the array width of the nozzles. Hereinafter, unless otherwise specified, explanation will be also given for other drawings in accordance with the similar rule.
Performing such a multipass printing suppresses an adverse effect to an image, and thus the image quality can be improved. This is because factors for deterioration of image can be reduced. Such factors include density unevenness caused by variation in the ink ejecting amount and the ink ejecting direction for each nozzle, and a stripe caused by insufficient conveyance accuracy of the printing medium between the main scannings (a white stripe if the conveying amount is excessively large, and a black stripe if it is excessively small).
The multipass printing method has such good characteristics, but is not effective enough to cope with displacement of ink landing positions in some cases. This will be hereinafter described.
For example, assume that the arranging pitch of the nozzles is 1/600 inch, i.e., approximately 42 μm (25.4 mm/600 dots), that is, the nozzle arranging resolution is 600 dpi (dots per inch). Here, consider two cases where the diameter of the ink dot having landed on the printing medium is approximately 32 μm and 42 μm.
Moreover, suppose that the conveying amount of the printing medium between the two main scannings is set to an integral multiple of the arranging pitch (42 μm) of the nozzles. Then, if the ink dot diameter is 42 μm, printing can be performed without any gap by aligning the ink dots in the sub-scanning direction. However, if the ink dot diameter is 32 μm, gaps exist between the ink dots aligned in the sub-scanning direction. In the former case, even if a slight conveying variation occurs in conveying the printing medium between the main scannings, the large ink dot diameters prevents change in the area factor from largely affecting a printed image. However, in the latter case, the printed image is sensitively affected by the change in the area factor.
FIG. 18 is an explanatory view of such a problem which occurs in the multipass printing. In the multipass printing, the sub-scanning (printing medium conveying) amount is usually set to an integral multiple of the nozzle pitch of the printing head. For this reason, downsizing of the ink dots results in gaps between the ink dots in the sub-scanning direction (that is, a region where printing cannot be performed, in this case, 10 μm). As apparent from FIG. 18, the nozzles for ejecting inks cannot be positioned so as to perform printing on these gaps even though the number of passes is increased. Therefore, when an error in the sub-scanning amount occurs, the printed image is sensitively affected by the change in the area factor. On the other hand, in the main scanning direction, the dots can be arranged without gaps by properly setting a timing of ink ejection during the print scanning.
For such problems, it is effective to use together a printing method called an interlace (Japanese Patent Laid-Open No. 10-157137(1998)) which is conventionally known as a high-resolution printing method.
FIG. 19 is an explanatory view of the inputted image data which is generated corresponding to the nozzle pitch (nozzle array resolution). The resolutions are 600 dpi for the main scanning direction and the sub-scanning direction, respectively. Printing of the image data with use of both the multipass printing and the interlace printing is as follows:
In the multipass printing, the printing medium is conveyed by the amounts equivalent to the width of the nozzle pitch (p)×n (where n is an integer equal to or greater than 0), and printing is performed while performing inter-pass complement with the width (hereinafter, also described as the “nozzle complementary width”). Use of the interlace printing together with the multipass printing means setting a conveying amount which is increased or decreased by the amount equivalent to 1/m (where m is an integer equal to or greater than 2) of the nozzle pitch from p×n, that is, a conveying amount which is a non-integral multiple of the nozzle pitch. In other words, the printing medium is conveyed relative to the printing head in the sub-scanning direction with use of the conveying amount of the nozzle pitch×(n+1/m) or the conveying amount of the nozzle pitch×(n−1/m) as appropriately, whereby printing is performed by increasing the printing resolution on the printing medium in the sub-scanning direction.
FIGS. 20A and 20B are explanatory views showing the case where the multipass printing is applied to the inputted image data shown in FIG. 19 and the case where the interlace printing is further applied to the data, respectively. Herein, for simplifying the explanation, a printing head on which 12 nozzles (seg 0 to seg 11) are arranged at a density of 600 dpi, and the 4-pass printing is performed for a unit region. Note that, the “unit region” refers to a region in which the inputted image data for 1 raster is printed. Particularly, in FIGS. 19, 20A, and 20B, the region formed by the width of the nozzle pitch (in the sub-scanning direction)×the printing width (in the main scanning direction) is defined as the “unit region”.
In the case where an image is formed by the 4-pass printing shown in FIG. 20A, the nozzle complementary width is the nozzle pitch p×3(=n=12/4). In the multipass printing, the printing medium is conveyed by the amount equivalent to the width thereof after each main scanning. Meanwhile, in the case of FIG. 20B, the printing medium is conveyed by the amounts equivalent to p×3, p×3.5, p×3, and p×2.5 repeatedly, while the number of divisions of the nozzle pitch width is 2 (=m). Herein, “m” can be rephrased to the “number of divisions in the vertical direction (sub-scanning direction) of 1 pixel of the inputted image data”.
Assume that addresses on the printing medium P are assigned with “A” to “H” in the horizontal position (main scanning direction position) and “1” to “12” in the vertical position (sub-scanning direction position). FIG. 19 is a schematic view of the inputted image data. The inputted image data of FIG. 19 is assigned to 1 pixel. Unlike FIG. 20B, no particular change for performing the interlace printing is made. In other words, the data a assigned to the pixel address A1 of the inputted image data should be printed to a printing pixel address A1 or A1.5 at a certain possibility in FIG. 20B.
In FIG. 20A, as a result of the sub-scanning (printing medium conveyance), the nozzles are set only at positions (hereinafter also referred to as raster) corresponding to integer pixel addresses (1, 2, 3, . . . ) in the vertical direction (sub-scanning direction). On the other hand, in FIG. 20B, the printing medium is conveyed by an amount equivalent to a number including a fraction on the decimal level (1/m). Accordingly, the nozzles can also be set at the positions with a fractional number (1/m) (that is, in the 1.5th raster, 2.5th raster, . . . ) in the vertical direction (sub-scanning direction). For example, in FIG. 20A, the nozzles seg 0, seg 3, seg 6 and seg 9 are set in the 1st raster of the vertical direction position to perform printing, and the nozzles seg 1, seg 4, seg 7 and seg 10 are set in the 2nd raster of the vertical direction position to perform printing. In this case, printing cannot be performed at the 1.5th raster position which is intermediate in the vertical direction. On the other hand, in FIG. 20B, printing at the 1.5th raster of the vertical direction can be performed.
As described above, in the case of FIG. 20B in which the interlace printing is used together with the multipass printing, the printing dots are arranged also at the sub-scanning direction positions, where printing cannot be performed in the case of FIG. 20A so as to increase the resolution, whereby robustness against displacement in the sub-scanning direction can be improved. Other various improvements have been proposed for the interlace (Japanese Patent Laid-Open No. 7-251513(1995), and Japanese Patent Laid-Open No. 11-034397(1999)).
However, the present inventors have found that a problem may occur depending on the relation between the number of passes of the multipass printing and increase in the resolution (the number of divisions m) in the sub-scanning direction through the interlace printing in the case where the interlace printing is used together with the multipass printing.
In FIG. 20B, printing is performed with use of the number m of divisions (=2) together with the 4-pass printing. Therefore, printing can be performed at the divided upper and lower rasters (that is, the positions with the integer addresses (1, 2, 3, . . . ) and the intermediate positions (1.5, 2.5, 3.5, . . . ) in the vertical direction) with 50% possibilities.
On the other hand, assume a case where the same input data is printed in the 3-pass printing, while setting the number m of divisions (=2), using the printing head with the same structure. In this case, the printing medium is conveyed by the amounts equivalent to p×3.5 and p×4.5, repeatedly. Then, as shown in FIG. 21, it is not possible to perform printing at the positions with the integer addresses and at the intermediate positions thereof at the equal possibility of 50%. Actually, the possibilities for both positions are respectively 33% and 66%. In addition, the appearance pattern of 33% and 66% is inverted at certain intervals.
FIG. 21 shows, substantial print scanning, total print allowing rates, and print images on the printing medium P in addition to the contents described in FIG. 20. Herein, the substantial print scanning represents that each raster existing in the vertical direction position is substantially printed by what number of the print scanning. For example, printing is performed on the first raster print position, by the first print scanning and the third print scanning. Since printing on the first raster cannot be performed by the second print scanning because the nozzle is at positions displaced by the amount equivalent to 0.5 pitch, the second print scanning is represented in parenthesis. As a result, the substantial print scanning is expressed as 1->(2)->3. On the other hand, on the 1.5th raster print position, contrary to the 1st raster, printing cannot be performed in the first and third print scanning since the nozzle is at the positions displaced by the amount equivalent to 0.5 pitch, and printing can be performed only in the second scanning. Therefore, the substantial print scanning is expressed as (1)->2->(3).
In the print scanning, in fact, printing is performed by applying a mask which specifies the print allowing rate for determining the arrangement of the print allowing pixels that are in complementary relation to the inputted image data. In the 3-pass printing, a mask is applied which enables printing one inputted image data at a possibility of ⅓ (=33%) in each scanning (hereinafter, the possibility that printing can be performed in each scanning is also referred to as the “print allowing rate”.). In this case, the total print allowing rate is calculated as the product of the print allowing rate and the number of substantial print scannings out of the total number of implemented print scannings (specifically, the number of print scannings except the print scannings in parenthesis). The number of substantial print scannings is 2 at the print position 1 since printing can be performed in the first and third print scannings, and it is 1 at the print position 1.5 since printing can be performed only in the second scanning. In other words, in the illustrated example, the total print allowing rates are 33% and 66% depending on their vertical print positions, and the total print allowing rates at the 1st and 1.5th raster print positions are 66% and 33%, respectively. Then, at the printing positions away from the 1st and 1.5th raster print positions by the amount equivalent to 1 nozzle complementary width (p×4), the pattern of the total print allowing rates 66% and 33% is inverted. Specifically, at the 5th and 5.5th raster print positions, the total print allowing rates are 33% and 66%, respectively. To facilitate understanding, in FIG. 21, the regions of the printing medium P with the total print allowing rate 66% are indicated in dark gray, and the regions with the total print allowing rate 33% are indicated in light gray. In this way, the total print allowing rates for the rasters are not the same, and the appearance pattern thereof is inverted at certain intervals.
The 3-pass printing is not the only printing in which the total print allowing rates are different depending on the vertical direction print position (raster). For example, also in the case where the 5-pass printing is performed, as shown in FIG. 22, it is understood that a print allowing rates are different depending on the rasters. In FIG. 22, a printing head in which 15 nozzles (seg 0 to seg 14) are arranged at a density of 600 dpi. In addition, the 5-pass printing is performed while setting the nozzle complementary width to the nozzle pitch P×3(=n=15/5), and the number of divisions of the nozzle pitch to 2 (=m). In this case, the printing medium is conveyed by the amounts equivalent to p×3.5 and p×2.5, repeatedly. Then, as apparent from the drawing, portions where printing is completed by three times of print scanning and portions where printing is completed by two times of print scanning are generated. As a result of this, the regions with the total print allowing rate 60% and the regions with 40% are mixed.
As described above, in the case where the inputted image data is divided into the image data corresponding to m (where m is an integer equal to or greater than 2) rasters that continue in the sub-scanning direction to perform printing in the interlace method, the number of the passes to be printed N (where N is an integer equal to or greater than 3) must be ‘a’ times of the number of divisions m (where a is a natural number). Otherwise, the following problems would be caused. In other words, as shown in FIGS. 21 and 22, print allowing rates are different among the print positions in the sub-scanning direction, and printing is performed while the print positions with different print allowing rates are mixed irregularly. These in turn cause stripe or unevenness on the image.